Snubbers are used to reduce turn-off stresses of controlled semiconductor switches. A capacitor in parallel with the switch to be protected reduces the rate of rise of voltage across the switch when the switch is turned off, absorbing energy that would have had to be dissipated by the switch. Conventional snubber circuitry introduces losses since the charge on the capacitor is dissipated through a resistor to prepare for the next switch turn-off. The controlled semiconductor switch carries the discharge current of the capacitor when the switch is on. If the switch to be protected is part of a power converter (a dc-dc or a dc-ac type, for example) and the converter operates at or slightly above the power utility frequencies, snubber losses can be maintained at acceptable levels using standard techniques. However, as the power levels and internal switching frequency of the power converter continues to increase, the resultant losses from conventional snubber circuitry proportionally increase. This is demonstrated by P=(1/2T)(.DELTA.CV.sup.2 /.DELTA.t) for a capacitive shunt snubber, where P is the snubber circuitry power dissipation, T is the switching period of the switch protected by the snubber, C is the value of the capacitor used in the snubber and V is the voltage across the capacitor in time .DELTA.t. The power dissipated is proportional to the frequency of operation. It can clearly be seen that as switching frequencies rise from 60 to 400 Hertz to 10 or 20 kHz, this loss cannot be justified. This is also especially true when the converter must operate and maintain good efficiencies at light loads. Furthermore, in addition to the loss in efficiency, there is a substantial increase in the cost of the dissipative components handling the increased energy waste.
An object of the present invention is to provide a low loss number, in which the snubber components are chosen for optimum stress relief.
A further object of the present invention is to provide a low loss snubber which occupies less volume than conventional snubbers.